The present invention relates to a three-layer headbox and to a method of applying a three-layer pulp slurry onto a paper machine forming section wire screen in order to produce a three-layer fiber web, in particular a paper web.
Three-layer headboxes with stationary blades between the top and bottom walls of the headbox for defining layers within the headbox are known. See, for instance, EP 0 581 051 A1. They have a problem in that different jet velocities for individual layers cannot be established at all or can be established only in part via built-in screens or only by variation of the amount or volume of suspension fed to the individual layers per unit of time. The construction of these headboxes is relatively simple, but their variability is also relatively slight.
DE-OS 37 04 462 also discloses a multi-layer headbox with blades which are swingable around their longitudinal axes between the individual nozzle spaces in order to establish speeds and pressures of the pulp slurry in the individual spaces, and the blades can be adjusted manually from the outside. This provides the advantage of great variability with respect to the adjustment of the differences in jet velocities. But it has the disadvantage that the manufacture of such a headbox is expensive, since a highly accurate mechanism per unit of time is required for adjusting the position of each blade.
There are several basic reasons for the need to control the speed of the individual layers of pulp slurry:
The formation of a paper web is a function of the differences in speed between the individual jets of pulp. The variation in the shearing forces between liquid suspension in layers which is possible in this way produces formation-affecting turbulences, which enable influencing the formation of the paper webs, if desired.
By variation of the jet velocity of an outer layer as a function of the size of the jet angle and of the jet/wire speed difference, the direction and length of the semi-axes of the breaking length ellipses of the paper web are influenced. They in turn correlate with the direction and statistical distribution of the fibers around the principal direction in the outer layer. Control of the mechanical properties of the web of paper is thus possible.
Upon drying, a web of paper undergoes preferred shrinkage in the direction transverse to the fiber layer, i.e., upon changes in moisture level and it is deformed in accordance with this property. If the fiber layer and the distribution in the outer layers of a web of paper are different, this favors so-called "curl" of a sheet, i.e. the tendency of a sheet of paper to curl upon changes in moisture. The tendency to curl can therefore also be influenced by changes in speed of the fiber layers.